A detailed description of the device used by the invention can be found in German Patent 27 30 941. The device is intended for the treatment of powdery, fibrous, pasty or pulpy goods and for the drying of the resulting granulate. Two different embodiments of the cutting heads are indicated therein. In the first embodiment, several rectilinear blades of various lengths are fastened to a shaft in several superimposed planes. In the second embodiment, the blades are bent upwards at their ends. As a result of the cooperation between the plowshare-like mixing tools and the cutting heads, the material is supposed to be pressed, in order to produce granulates, which can then be further condensed by the mechanical action of subsequently used tools.
The invention, however, has nothing whatsoever in common with the described use of the known device of the mentioned patent specification.
In the production of plasterboard, there is always a certain quantity of scrap which cannot be sold, consisting basically of damaged boards or boards which do not meet standards. The scrap results partially right after setting and partially during the drying of the boards. In the first case it contains 30 to 40% humidity, in the second case the scrap material is practically absolutely dry. The quantity of scrap material differs from manufacturer to manufacturer; as a rule it ranges between 2 and 8% of the produced amount and in extreme cases reaches even 10%.
In practice the scrap material is reintroduced into the production process. For this purpose, the boards are at first roughly preshredded. The preshredded material is a mixture of gypsum crumbs and of cardboard shreds up to the size of a palm, with clinging gypsum residues. In the most widely used plasterboards with a specific weight (area weight) of approximately 8 kg/m.sup.2 the paper content ranges between 4.5 and 5%, in plates with a specific weight of 11 kg/m.sup.2 between 3.4 and 3.8%. Up to now a separation of paper and gypsum--even, through screening,--which can possibly through screening--which can satisfy the practical requirements has not yet been technologically implemented.
The roughly preshredded goods cannot be processed by the usual calcinators. For instance in an indirectly heated rotary calcinator, the paper shreds can float on top of the gypsum grains and form clusters which cannot be reliably discharged and can clog the calcinator. There is also the danger when dry paper remains in the calcinator while it is emptied, that it can trigger an explosion when the calcinator is restarted.,
Also the roughly preshredded material has been fed already together with fresh gypsum to a mill. Under certain propitious conditions, the natural gypsum crumbs having an approximate size of up to 30 mm cause a certain further shredding of the paper shreds, but this is not always sufficient. When finely granulated flue gas gypsum or chemical gypsum are used as the raw material, a satisfactory further shredding cannot be achieved. Therefore this method has not been practical heretofore.